1966 Dodge Dart IRS Project Journal

About three years ago, I got this crazy idea to try and outfit my 1966 Dodge Dart with an independent rear suspension (IRS). I had been looking at a sales brochure for one of the new Chargers and it showed the IRS in those cars. It was contained in its own subframe, so it couldn’t possibly be hard to just take that and swap it into my Dart, right? Continue reading…

This blog will be a series chronicling my adventure. There will be about five posts depending on how long each section gets. I’m hoping to provide entertainment and inspiration if you would choose to follow my path. That being said, I can’t be responsible for your actions if you choose to build a suspension. If you can’t weld and your stuff breaks, it’s not my fault. There will be some required reading: Chassis Engineering by Herb Adams, Engineer to Win and Tune to Win by Carroll Smith. All of these books are practical guides that are easy to follow.

After you’ve read these books, I can recommend reading at Corner-carvers.comThis is a great web forum with lots of technical info that prides itself on not allowing stupid posts. Most of the time any questions you have will be answered without posting, if you just search. (If you post something there to the effect of: “Hai Bros!!1 What’s up?? Yer cars r da BOM!!!!11 LOLz” Your punishment will be swift, and I will personally find you and punch you in the face.)

This first post will be about finding a suitable donor car.

First off, let me introduce my Dart. It’s a 1966 GT with a 225 Slant Six. I’d previously upgraded the front end with stiffer springs, a sway bar, disc brakes, gas shocks and an aggressive alignment. I took it autocrossing a couple times and thoroughly enjoyed myself. But the car, although better than stock, still understeered pretty bad. I knew that the key to fixing this was fixing the rear end. The Dart had a solid axle with leaf springs, and although many people can make these handle well, the holy grail is an IRS. After I saw the new Charger IRS, the seed was planted.

My dream to use the Charger IRS was soon crushed when I started calling junkyards. I was on a serious budget and when I was quoted $3000 for the Charger rear end, I choked a bit. Another strike against the Charger rear was that no aftermarket gears are available and no sure-grip either.

So I started doing research. There’s not many IRS cars out there that are cheap. I knew that Corvettes and Vipers were IRS, but what else was there?  (I wasn’t looking at imports because I’m stubborn. But there are good choices there ie: Supra, Z car, Starion, RX7 etc.) While talking about this at work, a friend mentioned Ford Thunderbirds having an IRS.

I had no idea, but lo and behold, 1989-1997 Thunderbirds were IRS! And to top it off, if I could find a Thunderbird SuperCoupe, it would have an 8.8 with sure-grip and disc brakes.  This differential is very popular and strong. Lots of gears are available as well as sure-grips. This suspension also appeared in the Lincoln Mark VIII, but featured an all aluminum differential case. Below is a picture of the body style Thunderbird to look for.

So I started researching these cars and the more I saw, the more there was to like. Factory Five Racing uses parts from these cars and their design was a good reference for me. Below is a picture of their design.

Now at this point there were two ways I could go. I could cut my car up to fit the Thunderbird subframe or build my suspension around the car. I had some goals with the project that dictated that I build the suspension around the car. First of all, I wanted this rear end to bolt in with the option of going back to a standard axle later if I chose to. I also wanted to keep my stock gas tank, which ate up a lot of room underneath.

Building a suspension from the ground up opens a can of worms however. Suspension geometry has to be right to be safe. There’s a lot of math involved in getting it right. If you’d rather not worry about that and don’t mind cutting up your car, then just find a design that is close to the same width as your car and you’re good to go. I’m just a glutton for punishment I guess.

Anyway, I scored an entire IRS from a totaled SuperCoupe for $100. You can’t beat that.

That’s all for this post, join me next time when I finally get my hands dirty (and bloody)!